As an example of such a working vehicle, there has been known a skid steering loader in which running devices configured by tires or crawlers are provided on both left and right sides of the vehicle main body such that the traveling direction of the working vehicle is changed by making working speeds of the left and right running devices different. An arm supporting mechanism that is provided in the skid steering loader and vertically oscillates the arm is broadly classified into: a radial type supporting mechanism in which the arm is supported by pivotally connecting the base end part of the arm to the vehicle main body; and a vertical type supporting mechanism in which the arm is supported by pivotally connecting the base end part of the arm to the vehicle main body by using two links, the vertical type supporting mechanism being configured such that the vehicle main body, the arm and two links constitute a planar closed four-link mechanism in a side view. Generally, the vertical type supporting mechanism is configured by using the two links as described above. This supporting mechanism is referred to as the “vertical type” because many supporting mechanisms of this type have an arm front-end orbit which is substantially perpendicular to the ground as compared with the radial type.
Both U.S. Pat. No. 6,474,933 and U.S. Laid-Open Patent Publication No. 2007/0104566(A) disclose working vehicles, each equipped with a vertical type arm. Each working vehicle is configured to have the arm supported to the vehicle main body by two front and rear link members, and have the arm vertically oscillated according to expansion and contraction of a hydraulic cylinder arranged to stride over the arm and the vehicle body. In the case of the radial type supporting mechanism, the arm vertically oscillates around the pivotal connection point between the arm and the vehicle body. Therefore, the arm front end (and the attachment such as a bucket installed on the arm front end) vertically moves while drawing an arc centered on the pivotal connection point as viewed from the side of the vehicle. On the other hand, according to the vertical type, by changing the lengths of the two links and the pivotal connection positions to the arm and the vehicle body, a movement locus of the arm front end according to the vertical movement of the arm can be set relatively freely.
A loader of the skid steering type or the like (a loading working vehicle) has a bucket installed on the arm front end, for example, and is used for scooping out gravels and the like into the bucket and loading the gravels onto a carrying vehicle such as a dump truck. In the loading work, first, the arm is lowered and also the own vehicle is run and the bucket is oscillated, such that gravels are scooped into the bucket. Then, after the own vehicle is run to near the carrying vehicle such as the dump truck, the gravels are loaded onto the cargo stand on the carrying vehicle by running the own vehicle and oscillating the bucket while lifting the arm. When performing this loading work, in the case of the loader using the radial type supporting mechanism, the bucket installed on the arm front end is lifted while drawing an arc. Therefore, the bucket is lifted from the arm lowered position to a horizontal position while moving forward, and is lifted from the horizontal position to the arm-lifted position while moving back. In this case, the arm is located foremost at the horizontal position. Therefore, in order to avoid interference with the dump truck, the own vehicle needs to be located at a certain distance from the dump truck. However, in loading the gravels from the bucket onto the dump truck after the bucket is lifted, the bucket needs to be located on the cargo stand on the dump truck by moving the own vehicle forward. The bucket is retreated at the arm-lifted position as described above, the distance over which the own vehicle moves forward therefore becomes large. In the case of the loader using the radial type supporting mechanism, the bucket installed on the arm front end is lifted and lowered while drawing an arc forming a convex shape toward the front. Therefore, particularly when the arm is vertically oscillated in the state in which a heavy object is loaded in the bucket, there is a risk that the vehicle overturns forward.
On the other hand, when the vertical type supporting mechanism is used, the supporting mechanism can be configured to move the arm front end (and the bucket installed on the arm front end) so as to draw a substantially vertical locus when vertically oscillating the arm. By this arrangement, the arm can be vertically oscillated by bringing the own vehicle closer to the dump truck, and the forward movement distance of the own vehicle at the time of loading the gravels from the inside of the bucket onto the dump truck by lifting the bucket can be made small. Therefore, efficient loading work becomes possible. For the above reason, there have been used many working vehicles employing the vertical type supporting mechanism. Further, the vertical type supporting mechanism can be configured to move the arm front end (and the bucket) to draw a substantially vertical locus, as compared with the radial type supporting mechanism. This configuration capable of reducing the risk of the own vehicle overturning forward when the arm is vertically oscillated is one of the advantages of the vertical type over the radial type.
In U.S. Pat. No. 6,474,933, for example, discloses a vertical type supporting mechanism in which, when the arm is lifted from the lowered position, the arm front end draws an S shaped locus such that, after being lifted while slightly moving forward, the arm front end is further lifted while slowly moving back, and again lifted while moving forward in the middle. In the working vehicle according to U.S. Laid-Open Patent Publication No. 2007/0104566(A), the arm is configured such that, when the arm is vertically oscillated, the arm front end moves reciprocally longitudinally in an M shape.
In the case of employing the vertical type supporting mechanism, because of high degrees of freedom of the lengths of the two links and the hydraulic cylinder (the arm cylinder) and the arrangement position (pivotal connection positions to the arm and the vehicle body), how to set these items is important. Based on the setting of these items, the appearance design and the structure of the vehicle as a whole change, and also the movement locus of the arm front end changes. The appearance design and the structure are required to be simple as far as possible and to have a good appearance. Regarding the movement locus of the arm front end, considering the loading work described above, vertical movement of the arm front end is required. However, as in the working vehicle in U.S. Pat. No. 6,474,933, if the arm front end is located at a front position when the arm is oscillated to a high moving position, although the distance over which the own vehicle moves for loading onto the carrying vehicle becomes short, the vehicle receives the influence of the weight of the bucket cargo and becomes unstable at the time of lifting the arm. Further, in the case of U.S. Pat. No. 6,474,933, the bucket movement locus forms the S shape, and in the case of U.S. Laid-Open Patent Publication No. 2007/0104566 (A), the bucket movement locus forms longitudinal movement in the M shape. This has a problem in that the weight center of the vehicle changes longitudinally following the oscillation of the arm, and the working vehicle cannot be easily operated (has poor operability).
The present invention has been made in view of the above problems. An object of the present invention is to provide a working vehicle with excellent stability and operability of the vehicle at the time of vertically oscillating an arm, by allowing a movement locus of an arm front end following a vertical oscillation of the arm to be substantially vertical and by suppressing the movement amount in the longitudinal direction.